Thursday, March 20, 2008

The varmint was right!

On February 2, Punxsutawney Phil (the groundhog) predicted six more weeks of winter. And, by gum, he was right! It's now six-and-a-half weeks later, and spring has sprung (at least in the northern hemisphere). Today the sun will appear to move north of the equator, where it will stay for the next six months. At the north pole, the sun is rising, and at the south pole, the sun is set (stranding scientists at the South Pole Station until their spring comes).

Tomorrow is the full moon. In the Christian tradition, the first Sunday following the first full moon of spring is the Easter holiday (which explains why Easter is so early this year -- the full moon comes only a day after the start of spring). The Easter holiday remains one of the western calendar's few remaining links to lunar calendars (calendars based on the moon's 29-day cycle through its phases). Lunar calendars are still in popular use throughout the world -- the Islamic calendar, Jewish calendar, and Chinese calendar (among numerous others) are all based primarily on the moon's phase. But since the moon's phases don't match up exactly with the solar year (the length of time it takes the Earth to orbit the sun once, and the basis of the Gregorian calendar we use in everyday life), lunar calendars are a bit odd to the uninitiated, requiring the occasional extra month or something similar to bring the two back in line.

Astronomer's observations are often based on the lunar cycle. The full moon is very bright -- you will notice that you can see fewer stars in the sky when the moon is almost full as opposed to when the moon is absent from the sky. The faintest stars are hidden by the moon's glare. Even with telescopes, the moon's glare hides fainter objects. So, when we ask for telescope time, we have to specify if we can do our science when the moon is bright.

When the moon is bright, astronomers tend to look at bright stars, because their light cuts through the moon's glare. More recently, astronomers also tend to use the full moon time to look at objects in infrared light. Although the moon is still very bright in the infrared, the sky itself is always glowing brightly in the infrared, and the moon's light doesn't add much to the overall glow. (There are also stories of infrared astronomers moving the telescope to their next target and finding the moon is in the way!) But those of us who just want to study faint stars or galaxies in visible light are out of luck near the full moon.
So, even as the modern world steers further away from lunar calendars, observational astronomers still have to pay close attention to the phases of the moon.

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The Professor Astronomy Blog is based on work supported by the National Science Foundation under grant no. AST-0602288. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).